The invention relates to a circuit arrangement for protecting the input of an integrated circuit particularly suitable for analog signals, more specifically a CMOS circuit, against overvoltages, said circuit arrangement comprising:
at least two input terminals and
at least one protection stage in which each input terminal is connected to a power supply voltage by a respective protection diode, particularly a semiconductor diode, and to a reference potential by a respective protection diode, particularly a semiconductor diode.
Such a circuit arrangement is known from, for example, U.S. Pat. No. 4,254,442. This document discloses a circuit arrangement for protecting telephone connections and electronic circuits connected thereto against overvoltages; a first pair of semiconductor protection diodes is series-arranged in the same direction between two points of constant potential in such a way that the protection diodes are normally blocked, while the junction between the protection diodes is connected to a wire of the telephone connection so that, in the case of occurrence of overvoltages, the voltage of the wire is clamped on the potential of the one or the other point of constant potential. Furthermore, a second pair of semiconductor protection diodes is provided which is series-arranged in the same direction between two points of constant potential in such a way that the protection diodes are normally blocked, while the junction between the protection diodes is connected to the wire of the telephone connection; the wire of the telephone connection includes an impedance, preferably an inductance, between the junction with the first protection diode pair and the junction with the second protection diode pair, while the potential range between the points of constant potential of the second protection diode pair is larger than the potential range between the points of constant potential of the first protection diode pair or corresponds to this potential range.
The known circuit arrangements allow protection from overvoltages as occur, for example, in the bond pads (=electrically conducting contact areas of an integrated circuit, particularly a chip) and may destroy the subsequent integrated circuit (such overvoltages may occur, for example, in the case of ESD (=electrostatic discharge); particularly, the structure of CMOS circuits makes them very sensitive to high input voltages. The protection diodes become conducting when the input voltage decreases below the reference potential or exceeds the power supply voltage so that overvoltages are discharged via the protection diodes, i.e. are short-circuited to a certain extent by the protection diodes.
In other words, this means that the control range for the input voltage is limited to the voltage range between the reference potential and the power supply voltage. Such a limitation of the control range for the input voltage is, however, a problem when external high-frequency disturbances lead to voltage amplitudes at the input terminals which are outside the control range defined by the reference potential and the power supply voltage. In this case, the protection diodes change over to the forward direction, which involves demodulations of the mostly analog input signal; exactly such demodulation effects lead to significant disturbances in the signal path, also in the lower frequency range.
It is an object of the invention to provide a circuit arrangement of the type described in the opening paragraph, in which, on the one hand, disturbing demodulation effects affecting the signal path are reliably avoided in the input signal due to a simple and low-cost structure, and, on the other hand, effective protection against overvoltages caused by electrostatic discharges is ensured.
This object is solved by the characteristic features as defined in claim 1. Advantageous embodiments and essential improvements of the present invention are defined in the dependent claims.
In accordance with the teaching of the present invention, the control range for the input voltage can be increased to a voltage range which is increased by the breakdown voltages of the Z(ener) diodes and is thus larger than the voltage range between the reference potential and the power supply voltage by virtue of the fact that at least a further diode, particularly at least a Z(ener) diode and/or at least a preferably parasitic transistor diode with Z(ener) effect is arranged in series with each protection diode.
The further diodes are to be dimensioned in such a way that the voltage range increased by the breakdown voltages of the Z(ener) diodes must be within the voltage range, outside which range it would come to damage or even destruction of the integrated circuit to be protected. Furthermore, the further diode should be low-ohmic in the preferred way, i.e. it should have a low resistance value so as to cope with the short-lasting current flow (referred to as ESD current flow) caused by the overvoltage.
Due to the arrangement of at least one further diode in series with the protection diode, the integrated circuit (for example, CMOS circuit) may surprisingly receive a higher input voltage, particularly a higher alternating voltage amplitude without causing any harm to the subsequent integrated circuit. This input voltage, particularly the alternating voltage amplitude may fall below the reference potential by the breakdown voltage of the further diode(s) or may exceed the power supply voltage by the breakdown voltage of the further diode(s) without the protection diodes becoming conducting and without an unwanted demodulation effect caused by the (protection) diodes occurring.
It is true that it is known from EP 0 360 933 A1 to arrange two breakdown diodes in series or to arrange a breakdown diode and a Z(ener) diode in series. However, the document EP 0 360 933 A1 does not deal with the problem of coping with external high-frequency disturbances after passage through the protective stage, which disturbances lead to increased voltage amplitudes at the input terminals and which, based on the increased control range for the input voltage according to the invention, entail demodulation effects affecting the signal path at corresponding, high interference amplitudes.
In accordance with the teaching of the present invention, the protection stage precedes at least one filter stage, particularly at least one high-frequency filter stage. Undemodulated high-frequency interference signals are expressly damped by this filter stage so that the integrated circuit is substantially not affected by these interference signals.
In this respect, those skilled in the art will appreciate that a subsequent arrangement of such a (high-frequency) filter stage behind a conventional input stage of the type described in the opening paragraph (cf., for example U.S. Pat. No. 4,254,442) cannot filter out such a frequently occurring, demodulated, low-frequency signal in this arrangement in so far as this demodulated signal is approximately in the (low-)frequency range of the useful signal. In contrast, the (high-frequency) filter stage arranged subsequent to the protection stage maintains its full effectivity on the basis of the literally xe2x80x9cincreasedxe2x80x9d ESD protection in accordance with the teaching of the present invention.
The invention also relates to an integrated circuit, more specifically a CMOS circuit, comprising at least a circuit arrangement of the type described hereinbefore.